Many of the currently fuelded pyrotechnic munitions contain magnesium as a primary fuel, which is used with a host of other ingredients to produce light, sound, luminosity, or infrared emissions. Magnesium has many unique advantages as a pyrotechnic fuel, relative to other metal fuels. These include, for example, high reactivity, high heat of combustion and a low boiling point. Magnesium is available in both the atomized (spherical) and ground (oblong, ellipsoidal, or flake) forms.
However, despite these many unique advantages provided by magnesium as a pyrotechnic fuel, magnesium-fueled pyrotechnics, and to a lesser extent, aluminum fueled pyrotechnics, are also known to suffer from a very dangerous shortcoming. Specifically, they are very vulnerable to degradation by moisture during their life-cycle, resulting in the release, or out-gassing, of highly flammable hydrogen gas.
The solubility of magnesium oxide in acidic media, and the solubility of aluminum oxide in both acidic and basic media, are believed to contribute to this tendency to degrade in the presence of moisture. The degradation rate is more pronounced for magnesium because, relative to aluminum, the magnesium oxide coating is porous and comparatively non-protective. The high reactivity of magnesium, and the high particle surface area employed in these formulations also contributes to these undesirable degradation reactions for magnesium, especially with particle sizes in that are typically on the order of about 45-75 microns or less. Flammable hydrogen, together with magnesium hydroxide, is generated, for instance, through the following reaction: EQU Mg+2H.sub.2 O.dbd..dbd.&gt;Mg(OH).sub.2 +H.sub.2
The above reaction is accelerated when the pyrotechnic compositions are stored under conditions of high temperature and humidity, and is more vigorous when a relatively small particle size of magnesium is used. Generation of hydrogen (out-gassing) not only creates a highly explosive atmosphere, but also ruptures seals and cases in ordnance, resulting in severe safety, logistical, environmental, cost, and political consequences. In a further hazard, degraded ordnance often produces undesirable burning characteristics, resulting in poor performance. In addition, when metal fuel containing pyrotechnic munitions are severely degraded during prolonged storage in a hostile environment, the costs for demilitarization and disposal can be very significant.
As mentioned above, the art has been aware of these problems, and there have been previous attempts at preventing this hazard or containing the resulting hydrogen gas.
(1) Moisture control. Controlling the humidity level in the load plants and drying pyrotechnic ingredients, assembly components and packaging materials are two common methods employed in an effort to minimize pyrotechnic degradation by moisture. However, this is an expensive proposition, and given the need for the presence of some humidity (typically 40-50% relative humidity) to prevent electrostatic discharge ("ESD") or buildup, total dryness cannot always be achieved. PA1 (2) Alternative materials. Alternative fuels, such as powdered aluminum, have been successful for a limited number of pyrotechnic items such as the M115 and M116 simulators. For most purposes, however, aluminum cannot compare to magnesium in performance, e.g., burn times, rise times, spectral outputs, candle-powers, color values, etc. These shortcomings derive from the fact that aluminum has a significantly higher boiling point and lower reactivity, relative to magnesium. The other disadvantage of aluminum as a metal fuel, relative to magnesium, is as mentioned above, in contrast to magnesium, aluminum-containing alkaline metal nitrate oxidizer compositions are unstable in the presence of moisture. In addition, pyrotechnics prepared from aluminum particles of 16 microns or less, can also undergo an analogous degradation reaction, although at a slower rate, thus necessitating protective measures for aluminum, as well. PA1 (3) Packaging with barrier bags. Most pyrotechnic items are packed in barrier bags during manufacturing to prolong their shelf-life, but when moisture-induced out-gassing does occur, possibly due to pinhole-sized breaks in the barrier material, or due to pre-packaging exposure to moisture, the packaging can swell up with hydrogen gas, creating an additional safety hazard, so that shipping such materials requires special precautions and waivers from the U.S. Department of Transportation. PA1 (4) Release of hydrogen gas. Personnel in the fuelds or at depots cut holes through the bulging barrier bags with knives to release hydrogen to the atmosphere and then repack the ordinance. Unfortunately, this has lead to at least one reported instance of fire and injury to personnel. PA1 1. The components of the coated metal fuel powders are more uniformly and intimately mixed by the inventive processes, resulting in powdered metal fuels that are more uniformly coated, with improved resistance to environmental moisture relative to pyrotechnics produced by previous methods. PA1 2. The coated metal fuel powders produced by the inventive processes are reproducible in their desirable properties. For example: resistance to moisture-induced degradation and hydrogen outgassing, ballistic performance, and mechanical strength. The coated metal fuel powders are also ready for loading and assembly to mass produce pyrotechnic devices, and will not segregate to jeopardize performance of the pyrotechnic product. PA1 3. The inventive processes are environmentally safer, more efficient and less costly, e.g., by avoiding the use of toluene and substituting safer and lower boiling point solvent(s). PA1 4. The coated metal fuel powders are more free-flowing, compared to previously prepared products, and less dusty, for improved handling and reduced fire-hazard during manufacturing.
There have also been efforts to render particulate metal fuels, such as magnesium, hydrophobic by coating the particles with organic resins. For example, several authors have described the use of ethylene and vinyl acetate co-polymers to render metal fuel particles hydrophobic and resistant to moisture induced degradation. These resin copolymers have a desirably high tensile strength and produce a protective hard surface that minimizes abrasion. Ethylene and vinyl acetate co-polymers are commercially available under the tradename of Elvax.RTM. (E.I. Dupont De Nemours & Company, Wilmington, Del.). Elvax.RTM. is available in a number of grades and weights, including the 40W, 150W, 240W, 265W and 360W formulations, among others.
Taylor et al., 1987 ("Organic Coatings to Improve the Storageability and Safety of Pyrotechnic Compositions," Technical Report ARAED-TR-87022, US Army Research, Development and Engineering Center, Picatinny Arsenal, N.J., USA), prepared Elvax.RTM. 360 coated magnesium powder by a method that included stirring magnesium powder into a 5% solution of Elvax.RTM. 360 dissolved in toluene. L. V. Yong, 1992, ("Corrosion Production of Magnesium Powder in Pyrotechnic Compositions," Australia, Eighteenth International Pyrotechnic Seminar, 13-17 July) reported magnesium particles coated with Elvax.RTM. dissolved in toluene, using a slurry coating technique.
The processes proposed by both Taylor et al. and by Yong, employ toluene, which is extremely toxic and is on the EPA priority HazMats list of extremely toxic or carcinogenic chemicals. In addition, toluene has a higher boiling point (110.degree. C.), relative to other solvents used in pyrotechnic productions, e.g., alcohol. However, Yong failed to provide methods for manufacturing tetrafluoroethylene (Teflon.RTM.)--containing pyrotechnic compositions. Taylor et al. appears to have described only dry blending techniques to prepare Elvax.RTM.--containing magnesium-Teflon flare compositions. It should be noted that Yong did not test 240W Elvax.RTM. and Taylor et al. did not test 40W, 15W, and 240W grades of Elvax.RTM.. Regardless of the techniques or processes used by Yong and Taylor et al., products produced by those methods have not solved the longstanding out-gassing problem as recognized by the art.
Thus, there remains a need in the art for improved methods and compositions for producing metal-particle pyrotechnic compositions which have the following desirable properties. Further, there remains a need in the art for a scalable granulation process for producing magnesium and/or aluminum pyrotechnics that are successfully protected by Elvax.RTM..